Discussion in 'Boat Design' started by tom kane, May 6, 2015.

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### tom kaneSenior Member

A discussion about the design of a boat for shallow draft and the information
What they are teaching is not my theories.

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### WaterwitchSenior Member

Ok I actually went through the lessons, they are teaching the same lessons I learned in the 7th grade. Buoyancy equals displacement, a volumetric measurement. You on the other hand seem to confuse hydrostatic pressure, and wetted surface somehow being responsible for buoyancy.

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### RastapopNaval Architect

Well it is technically that pressure integrated over the surface area that gives us buoyancy...

But you're right, Tom's understanding is incorrect. Tom, the buoyancy force is, must, and always will be, equal to the weight of the water displaced (in static equilibrium). In other words, displaced water volume is everything, regardless of shape. Density changes are so small as to be insignificant at the depths we're talking about.

And some of us here are engineers, so why you think high school physics may be of interest is a mystery.

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### Mr EfficiencySenior Member

Quite remarkable that this conversation is even taking place !

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### PARYacht Designer/Builder

. . . . .

What I'd like to see is a 15' x 6' flat bottom boat of say 1,600 pounds displacement and a 15' x 6' V bottom of 1,600 pounds displacement. Lets see the profile of their respective LWL's at this displacement. From experience I can tell you that given the same physical dimensions, the same keel rocker, etc. a 15' x 6' flat bottom, will displace in the 25 cubic foot range, while a V bottom of the same dimensions will be 25% - 30% less or there abouts, making this hypothetical 15' V bottom displace about 19 cubic feet. This assumes a 6" draft on both hulls. I've designed over a dozen boats of these approximate dimensions and of course the type and amount of V will vary the volume considerably, but the above example is indicative of a modest warped bottom. If the V hull was forced down to the same volume of the flat, it'll draw about 7 1/2", maybe 8".

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### tom kaneSenior Member

Attention class!
We have two problems, no 1,many of you are making assumptions that claims have been made on this thread on certain subjects.

No 2 is we have a boat plan we wish to change to the customers needs.
The only information we have is the diagrams in the image.
We need a boat that has as good or better possible draft as the boat in the image and built of wood.
And of similar or smaller length and possible beam,lighter and easy to trailer.
The boat is to be used for family camping,fishing,water skiing and general family fun. The boat must be robust and stand dragging up beaches.
The boat must be capable of being used to sleep two and live aboard.
Propulsion main unit to be a four cylinder petrol inboard closed water cooling.
The boat must be safe from any possible gas explosions.
Other needs to be discussed with customer. The customer is not concerned what it looks like just utility.

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### PARYacht Designer/Builder

So now you're listing an SOR, instead of the original debate about which hull form floats deeper? Why answer the question or defend your position, when you can just misdirect or make it convoluted? Do you think the linked program would be the first time something was taught incorrectly? In parts of Texas they're removed all traces of Darwin's efforts in textbooks, because well you know, he never existed, so couldn't have been correct about evolution. This doesn't mean they're right, just ignorant.

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### rasorincSenior Member

Tom, for max safety for engine and fuel tank go with a diesel engine. It is the only way I know for total piece of mind. Do that and vent well assures you of no explosion.

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### RastapopNaval Architect

I know this isn't a case of misunderstanding PAR, just a "typo" if you will, but you've said here a boat of 1,200 lb can displace 25 cubic feet, which is impossible (assuming it's floating, on seawater).

Like I said, I realise it isn't that you misunderstand, it's just that you've written something other than what you meant.

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### PARYacht Designer/Builder

Oops, 25 cubic feet is about 1,600 pounds. 19 cubic feet is about 1,200 pounds, which is where I likely crossed up my fingers. Good catch . . .

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### tom kaneSenior Member

12. Joined: Nov 2003
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### tom kaneSenior Member

Possibly a good idea for some people but the customer wants a petrol engine.
Lighter, cheaper etc.,

13. ### yofishPrevious Member

I'm confused. Am I reading that two hulls of equal displacement (weight) will somehow displace less weight only because of a difference in shape?

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### tom kaneSenior Member

Check your comments with what is on the website,more is involved such as water pressure (uplift) acting from all directions on an object.

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### RastapopNaval Architect

No, just an error, see posts #9 and #10. (Maybe you could edit a correction in that post PAR?)

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